Various imaging techniques have been developed to image internal characteristics of an object of interest based on detecting one or more properties of the object. Applications include, but are not limited to, medical imaging to facilitate detection, diagnosis and/or treatment of biological anomalies within a subject patient, and security applications such as, detection of contraband, explosives or other prohibited subject matter in postal packages, passenger baggage, etc. Exemplary imaging techniques include x-ray computed tomography (CT), magnetic resonance imaging, electrical impedance tomography, ultrasonography, etc.
Magnetic resonance imaging (MRI) includes techniques for capturing images of the internal structure of an object of interest, for example, by non-invasively obtaining images of internal structure of the human body, and has been widely used as a diagnostic tool in the medical community. MRI exploits the nuclear magnetic resonance (NMR) phenomenon to distinguish different structures within an object of interest. For example, in biological subjects, MRI may be employed to distinguish between various tissues, organs, anatomical anomalies (e.g., tumors), and/or to image blood flow, blood perfusion, etc.
In general, MRI operates by manipulating spin characteristics of subject material. MRI techniques include aligning the spin characteristics of nuclei of the material being imaged using a generally homogeneous magnetic field and perturbing the magnetic field with periodic radio frequency (RF) pulses. To invoke the NMR phenomenon, one or more resonant coils may be provided proximate an object positioned within the magnetic field. The RF coils are adapted to generate RF pulses at a resonant frequency that matches a Larmor frequency of certain material within the object to excite the nuclei and cause the spin to briefly precess about an axis in the direction of the applied RF pulse, rather than in the direction of the applied magnetic field. The Larmor frequency is related to the rate at which a nuclear spin precesses about an axis, which is, in turn, proportional to the strength of the applied magnetic field. When the RF pulse subsides, the spins precess about and gradually realigns with the magnetic field, releasing energy that can be measured and used to form one or more images of the internal structure of the object being imaged.
The development of MRI imaging devices, referred to herein as MR scanners, has tended toward implementations that use higher magnetic field strengths. One benefit of higher magnetic field strengths includes a proportional increase in the signal-to-noise ratio (SNR) of the NMR signal emitted from a target region. In particular, the ratio of NMR signal strength to noise increases in an approximately linear fashion with increased magnetic field strength. This increase in SNR allows higher resolution images to be obtained using higher magnetic field strengths. The clinical standard for MRI scanners is poised to shift from 1.5 Tesla to 3 Tesla (T). MRI scanners capable of imaging a human body using a magnetic field of 7 T or higher are currently being tested at various research centers. In general, high field MRI refers to MR scanners operating at 3 T and above.
Spatial encoding of NMR signals emitted from a subject has traditionally been achieved by applying magnetic field gradients to localize the NMR effect. However, recent trends in MRI involve achieving at least some of the spatial encoding conventionally accomplished with magnetic field gradients by providing multiple and parallel RF transmit and/or receive coils. The MR technique of using multiple transmit and/or receive coils to image a subject is referred to herein as parallel MR. Parallel MR takes advantage of the spatial information available using an appropriately arranged array of RF coils, as discussed in further detail below.
In parallel MR, some number of the RF coils in an array may be independently excited (e.g., RF power may be transmitted over independent channels to multiple respective RF coils) and/or independently measured (e.g., measurements may be obtained/received from multiple RF coils over respective independent channels). Parallel MR has circumvented previous limits on speed and efficiency, effecting a reduction in image acquisition times and improving the spatial resolution of acquired images for any give acquisition time. As a result, parallel MR is becoming a generally important part of many modern MRI scanners. Manufacturers of MRI equipment are engaged in designing and producing ever larger numbers of independent channels for data transmission and reception.
Another method of obtaining properties from an object of interest includes examining changes in the resonant properties of RF coils in the presence of the object. When a resonant coil is placed in proximity of a load, for example, a patient or other object to be imaged, various properties of the resonant coil are affected. In MRI, this loading effect tends to negatively impact the operation of the device by shifting the resonant frequency of the coil and/or causing other generally undesirable changes in coil properties that may lead to reduced sensitivity and decreased efficacy in the coil's ability to appropriately induce the NMR effect. This loading effect depends in part on dielectric properties (e.g., conductivity, electrical permittivity, magnetic permeability, etc.) of the load and therefore varies with the loading object.
The effects of coil loading complicate MRI, since uncompensated shifts in the resonant properties of transmit and/or receive coils can significantly degrade image quality. In fact, resonant coils are often tuned or adjusted to compensate for the generally undesirable loading effect caused by the object being imaged, and coil designers must often sacrifice optimal detector performance in favor of overall robustness for a variety of loading conditions. However, the sensitivity of resonant coils to loading effects may be viewed as a detection mechanism, rather than an inconvenience, to effectively measure dielectric properties (e.g., conductivity, permittivity, permeability, etc.) of an object proximate to the coil(s). In particular, a change in resonant properties of one or more resonant coils due to loading may provide information about the distribution of dielectric characteristics of the loading object.
The imaging modality using changes in coil resonance to characterize one or more dielectric properties of a loading object is referred to herein as radio frequency impedance mapping (RFIM), and describes generally any of various methods of mapping resonant properties and/or impedance characteristics of one or more resonant coils to dielectric properties of a body coupled to the one or more resonant coils. Details of this imaging modality and are described in Patent Cooperation Treaty (PCT) Publication WO2004/026136 ('136), entitled “Radio Frequency Impedance Mapping,” and U.S. application Ser. No. 10/527,592 ('592), entitled “Radio Frequency Impedance Mapping.”
In brief, one RFIM approach involves transmitting an RF signal along each of a plurality of coils in a resonant RF coil array and measuring the received responses in all of the other coils in the array. Such measurements may be performed first in the absence, and then in the presence, of an object to be imaged. The measurements may then be compared with a computational model of the coil array and the imaged object. Values may be chosen for one or more of conductivity, permittivity, and permeability of each region of the object. The model may then be adjusted until it best matches the experimental data obtained from the RF coil measurements. The result is a map (e.g., an image) of the spatial distribution of dielectric properties throughout the object.